In recent years, it has become necessary that image display devices have high-resolution and high picture quality, and it is desirable for such image display devices to have low power consumption and be thin, lightweight, and visible from wide angles. With such requirements, display devices (displays) have been developed where thin-film active elements (thin-film transistors, also referred to as TFTs) are formed on a glass substrate, with display elements (for example, organic light-emitting diode layers to produce light, or liquid-crystal layers to block light from a backlight) then being formed on top.
A problem with displays combining white-emitting devices with color filters is that the combination of emitter and color filters must provide a good color gamut for the reproduction of a wide range of colors. Color filters used in this way must have good spectroscopic characteristics, with sufficient transmittance with the predetermined visible light region and no unnecessary transmittance in other regions of the visible spectrum. This has been difficult to achieve. The blue filter of a Sony Qualia television suffers from significant transmittance in the green region of the spectrum, which is not desirable for high blue color purity.
Much work has been done to identify good color filters and color filter combinations for liquid crystal displays (LCD), e.g. “Liquid Crystal Displays”, Ernst Leudner ed., John Wiley & Sons (2001), pp. 28-296; “High Performance Pigments”, Hugh M. Smith, John Wiley & Sons, pp. 264-265; Kudo et al., Jpn. J Appl. Phys., 37 (1998), pp. 3594-3603; Kudo et al., J. Photopolymer Sci. Tech. 9 (1996), pp. 109-120; Sugiura, J. of the SID, 1(3) (1993), pp. 341-346; FU et al., SPIE, Vol. 3560, pp. 116-121; Ueda et al., U.S. Pat. No. 6,770,405; and Machiguchi et al. U.S. Pat. Nos. 6,713,227 and 6,733,934.
Despite such improvements, display color reproduction has remained full of compromises. For example, the standards for color television gamut, as described by Fink in “Color Television Standards”, McGraw-Hill, New York (1955), and in Recommendation ITU-R BT.709-5, “Parameter values for the HDTV standards for production and international programme exchange”, have seldom been met. The former NTSC reference describes a good red primary as having 1931 CIE x,y chromaticity coordinates of x=0.67 and y=0.33, while a good green primary has coordinates of x=0.21 and y=0.71. The latter HDTV reference defines a good blue primary as the original PAL/SECAM blue having coordinates of x=0.15 and y=0.06. Commercially available televisions fall short of these standards and have a compromised color gamut. Takizawa, in US 2004/0105265, teaches a red filter that can achieve an x value as high as 0.65 and a y value as high as 0.33, which falls short of the NTSC reference red primary in x. Yamashita, in U.S. Pat. No. 6,856,364, teaches a red filter that can achieve an x value as high as 0.665 and a y in the range from 0.31 to 0.35. While this is an improvement over Takizawa, a red primary that meets or exceeds the x value of the NTSC primary would result in a purer red color. Yamashita further teaches a blue filter wherein the x value can range from 0.13 to 0.15 and the y value can only be as low as 0.08, and a green filter wherein the x value can range from 0.22 to 0.34 with a y value ranging from 0.56 to 0.65. Both of these fall short of the respective desired primary x,y values, which if achieved would result in purer blue and green colors, respectively.
Additionally, liquid crystal displays commonly available often use a backlight such as a cold-cathode fluorescent light (CCFL). It is a characteristic of CCFL sources commonly available that, while it provides white light consisting of a variety of wavelengths of the visible spectrum, the light is often more intense in a few narrow bands of the spectrum. These bands are generally centered in the red, the green, and the blue regions of the spectrum. The color filters needed with such light sources do not need to be especially narrow to provide a good color gamut. For example, a red filter can allow a transmission “tail” into parts of the green region of the spectrum, so long as the tail region does not include the major green emission peak, and still provide good color with such a light source.
Organic light-emitting diodes (OLEDs) provide another light source for displays. Unlike LCDs, which have a single full-display light source, OLED displays only produce light at the pixels that are required to be bright at a given time. Therefore, it is possible for OLED devices to provide displays that have reduced power requirements under normal usage. There has been much interest in broadband-emitting OLED devices in color displays. Each pixel of such a display is coupled with a color filter element as part of a color filter array (CFA) to achieve a pixilated multicolor display. The broadband-emitting structure is common to all pixels, and the final color as perceived by the viewer is dictated by that pixel's corresponding color filter element. Therefore, a multicolor or RGB device can be produced without requiring any patterning of the emitting structure. An example of a white CFA top-emitting device is shown in U.S. Pat. No. 6,392,340. Kido et al., in Science, 267, 1332 (1995) and in Applied Physics Letters, 64, 815 (1994), Littman et al. in U.S. Pat. No. 5,405,709, and Deshpande et al., in Applied Physics Letters, 75, 888 (1999), report white light-producing OLED devices. Other examples of white light producing OLED devices have been reported in U.S. Pat. No. 5,683,823 and JP 07,142,169.
One property of broadband OLED displays is that, while they can vary somewhat in emission intensity at different wavelengths, they generally do not have the intense peaks characteristic of CCFL sources. Therefore, common color filters that provide adequate color gamut when coupled with a CCFL display may not provide good results with OLED displays. The example above of a red color filter with a “tail” into a portion of the green region of the spectrum can provide an adequate red emission for a CCFL source, but be totally unsuitable for use with an OLED device.
Therefore, it is a problem to be solved to produce color filters that can be coupled with broadband OLED devices to provide displays with improved color rendition.